x86: Make debug UART port configurable via system config

x86: Make debug UART port configurable via system config

We already allow to enable a VGA console via the system config, so let's
make the UART port configurable this way as well: phys_start will hold
the port, and flags must not have JAILHOUSE_MEM_IO set, in order to
differentiate us from the memory-mapped VGA console. And by leaving
phys_start at 0, we can even turn off the console now.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>